In the present work, interspecies electron transfer (IET) between fermentative and electro-active species was investigated using Geobacter sulfurreducens and Clostridium pasteurianum as model microorganisms in co-culture experiments1. In a fermentation medium containing glycerol and acetate as substrates, G. sulfurreducens was able to grow using C. pasteurianum as sole electron acceptor. As a result, C. pasteurianum fermentation pattern was significantly altered compared to pure culture controls. Indeed, 1,3-propanediol and butyrate production were both improved at the expense of butanol and ethanol. Interestingly, C. pasteurianum growth yield decreased by almost 40% in co-cultures despite a theoretical ATP production (i.e. calculated from metabolic patterns) being 8.4% higher in co-culture compared to C. pasteurianum pure culture. To better understand such drop in biomass yield, energetics of IET between electro-active and fermentative species was theoretically explored using a thermodynamical model. Although this approach could predict an IET-mediated parasitism when fermentative species acts as electron-accepting microorganism, it failed to explain the drastic biomass yield decrease observed experimentally, thus suggesting that more complex biological regulations were implicated. While fundamental principles are still to be elucidated, IET-mediated parasitism could be a promising way to optimize carbon recovery during fermentations by both reducing biomass production and improving metabolite yields of value-added compounds such as 1,3-propanediol.